fix: preserve baseUrl path in WebSocket URL construction

[sanderdw]

Summary

The connect() method was discarding the path component of baseUrl when building the WebSocket URL, using only url.host. This breaks reverse proxy setups where Sendspin is served behind a subpath.

Fixes #91

Changes

Two small changes in connect():

1. Support relative URLs — pass window.location.href as the base argument to new URL(), so relative paths like ./sendspin-proxy/ resolve correctly against the current page
2. Preserve the path — include url.pathname when constructing the WebSocket URL, so proxy paths are not stripped

Before

const url = new URL(this.config.baseUrl);
this.wsUrl = `${wsProtocol}//${url.host}/sendspin`;

baseUrl	WebSocket URL
http://192.168.1.100:8927	ws://192.168.1.100:8927/sendspin ✅
https://example.com/proxy/	wss://example.com/sendspin ❌ (path lost)
./sendspin-proxy/	💥 TypeError: Invalid URL

After

const url = new URL(this.config.baseUrl, typeof window !== "undefined" ? window.location.href : undefined);
const basePath = url.pathname.replace(/\/$/, "");
this.wsUrl = `${wsProtocol}//${url.host}${basePath}/sendspin`;

baseUrl	WebSocket URL
http://192.168.1.100:8927	ws://192.168.1.100:8927/sendspin ✅
https://example.com/proxy/	wss://example.com/proxy/sendspin ✅
./sendspin-proxy/ (on https://ha.local/app/)	wss://ha.local/app/sendspin-proxy/sendspin ✅

Backward Compatibility

Fully backward-compatible — absolute URLs without a path still produce the same WebSocket URL as before, since pathname is / which gets stripped by the trailing slash removal.

[maximmaxim345]

LGTM. Thanks @sanderdw !